Russian and Syrian warplanes have intensified attacks on rebels who over-ran most of Aleppo, Syria’s second city, in a lightning assault that poses the biggest challenge in years to Bashar al-Assad’s regime.

Air raids struck the rebel-held city of Idlib for a second day on Sunday, while opposition media and war monitors said Russian and Syrian jets had also launched attacks near Aleppo University Hospital.

Thousands of rebels, led by Islamist group Hayat Tahrir al-Sham, entered Aleppo city, which has a population of 2mn, on Friday. Images circulating on opposition-linked social media this weekend showed them raising their flag over the city’s citadel and posing in its airport.

The rebels, who launched their assault only on Wednesday, said their fighters had advanced in multiple directions from their stronghold in Idlib province in north-western Syria, although their progress seemed to have slowed by Sunday.

HTS rebels attempted to press on to the major regime-held city of Hama, south of Aleppo, and claimed they had seized at least four towns in Hama province. The Syrian army has denied this. Rebels also said they captured the strategic town of Sheikh Najjar.

In his first public comments since the start of the offensive, Assad said Syria would continue to “defend its stability and its territorial integrity in the face of terrorists and their supporters”, in remarks carried by state news agency Sana.

The comments came during a call with Emirati leader Mohammed bin Zayed, an Assad ally, who “emphasised the UAE’s solidarity with Syria and its support in combating terrorism”.

Later, Assad vowed to defeat the insurgents in a phone call with the acting leader of the breakaway Georgian region of Abkhazia, Badra Ganba, saying “terrorism only understands the language of force”.

It was not clear whether Assad had returned to Syria following a visit to Moscow earlier this week.

The Syrian army denied that the rebels had secured Aleppo, but later said it was redeploying its forces as it prepared to launch a counterattack supported by Russian air strikes and strengthen its defensive lines after days of fierce fighting. Dozens of Syrian army soldiers were killed by rebel forces, the defence ministry said.

Social media videos showed rebel fighters driving through the streets of Aleppo, pulling down and kicking statues of Assad family members and celebrating by honking horns and firing their weapons. Videos also showed them freeing captives from Aleppo prisons.

Hundreds of civilians fled the city and its suburbs and headed for regime- or Kurdish-controlled areas, fearing a repeat of the gruelling 2016 battle that devastated their city. Those who remained in Aleppo were placed under night-time curfew by the HTS, residents told the Financial Times, adding that the streets were mostly empty on Sunday.

Assad faces increasing domestic and external pressures in a country shattered by a civil war that erupted after a 2011 popular uprising. He was able to quash the original rebellion with military backing from Russia, Iran and Iran-backed groups, including Hizbollah, the Lebanese militant movement.

Despite regaining control over two-thirds of the country, years of conflict and a deep economic crisis have left much of Syria in ruins.

The fighting had largely subsided in recent years, with the surviving rebel groups pushed into northern and north-western areas close to the Turkish border.

But over the past year, Israel has intensified air strikes on Iran-affiliated targets in Syria as it launched an offensive against Hizbollah in Lebanon, severely weakening groups that had played a vital role in keeping Assad in power.

HTS’s ability to move deeper into Syria is a major embarrassment for Assad, underscoring the regime’s weakness. The offensive appeared to have been planned for years, and comes at a time when Assad’s allies are preoccupied with their own conflicts in Ukraine and the Middle East.

HTS leader Abu Mohammad al-Jolani said his fighters would not rest “until we reach the heart of Damascus”, in old video footage that was republished by social media networks linked to the group this weekend.

Russian warplanes bombed rebel positions in a bid to stem their advance. Russia’s defence ministry was quoted by state news agencies as saying the country’s forces had killed “at least 300 militants by missile strikes . . . on command posts, warehouses and artillery positions”.

Russian military blog Rybar, known to be close to the defence ministry, said it understood that Major General Sergei Kisel, the top commander of Russia’s forces in Syria, had been removed from his post.

He was earlier removed from his position commanding Russia’s 1st tank army in Ukraine shortly after the start of Russia’s full-scale invasion in 2022, after the supposedly elite force suffered heavy defeats in the first weeks of the war.

The Rybar channel also said Russian troops were forced to evacuate the Kuweires air base in the Aleppo area as the rebels advanced. The Syrian air base was regularly used by Russian forces.

Russian foreign minister Sergei Lavrov has discussed the situation in Syria with Hakan Fidan, his Turkish counterpart. Lavrov also spoke to Iran’s foreign minister Abbas Araghchi, who is expected to visit Damascus on Sunday and Ankara on Monday, as the main powers involved in Syria began a flurry of diplomacy.

Araghchi on Sunday reaffirmed Iran’s unwavering support for Assad, accusing radical Islamist factions of aligning with the interests of the US and Israel. “We will witness their defeat,” he said.

Iranian state media reported that opposition forces in Aleppo seized Iran’s consulate on Saturday, tearing down and destroying images of Iranian political and military leaders.

The Kurdish-led Syrian Democratic Forces, which are supported by the US in the fight against Isis and control swaths of Syria’s north-east, announced a general mobilisation on Sunday. They called on people to join in the defence against the rebel offensive, which they say was “orchestrated” by Turkey — their longtime foe.  

Additional reporting by Najmeh Bozorgmehr in Tehran

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President-elect Donald Trump has said he will appoint Kash Patel, a loyalist and hardline critic of the “deep state”, to lead the FBI, signalling he will seek to remove Christopher Wray as head of the agency. 

Patel, who advised the secretary of defence under Trump’s previous administration, has suggested carving out the FBI’s intelligence-gathering function and purging its ranks of employees who do not support Trump. He has also mused about retribution against Trump’s critics.

“I am proud to announce that Kashyap ‘Kash’ Patel will serve as the next Director of the Federal Bureau of Investigation,” Trump wrote on his social media website, Truth Social, on Saturday night.

“Kash is a brilliant lawyer, investigator, and ‘America First’ fighter who has spent his career exposing corruption, defending Justice, and protecting the American People.”

“This FBI will end the growing crime epidemic in America, dismantle the migrant criminal gangs, and stop the evil scourge of human and drug trafficking across the Border,” Trump added. 

A longtime Trump loyalist, Patel has also worked as a federal prosecutor and public defender, but does not have as broad a law enforcement experience as many FBI directors. 

He has railed against an alleged “two tier system of justice” he has claimed is “the deep state’s weapon of choice”. The federal criminal case that accused Trump of mishandling classified documents was “the best definition” of such a system, Patel told rightwing podcaster Shawn Ryan earlier this year.

The DoJ is seeking to drop the case, which was dismissed by a federal judge, due to an internal policy that bars prosecution of a sitting president.

In a podcast hosted by Trump ally Stephen Bannon late last year, Patel vowed to investigate and “come after” journalists who “lied” and “helped Joe Biden rig presidential elections.”

“Whether it’s criminally or civilly, we’ll figure that out,” Patel told Bannon. 

In his book Government Gangsters, Patel outlined a list of “top reforms to defeat the deep state” — the supposed permanent government of left-leaning bureaucrats that Trump and his allies believe worked against his first administration.

“The FBI’s footprint has gotten so freaking big, and the biggest problem the FBI has had has come out of its intel shops,” Patel told Ryan. “I’d break that component out of it”.

Patel also vowed to “shut down” the FBI’s historic headquarters in Washington “on day one and reopen it the next day as a museum of the ‘deep state’”.

He would also “take the 7000 employees that work in that building and send them across America to chase down criminals,” Patel said. “Go be cops, you’re cops”.

Before joining the administration during Trump’s first term, Patel worked as a staff member for the House Permanent Select Committee on Intelligence under Republican congressman Devin Nunes, helping run the committee’s investigation into Russia’s interference in the 2016 campaign.

Trump’s effort to place Patel at the head of the US’s premier law enforcement agency will require Senate confirmation.

Trump appointed sitting FBI director Christoper Wray in 2017, and his term does not expire until 2027. Trump has been openly critical of Wray, particularly after law enforcement officials searched his residence in search of classified documents.

A spokesperson for the FBI said that Wray’s “focus remains on the men and women of the FBI, the people we do the work with, and the people we do the work for.”

“Every day, the men and women of the FBI continue to work to protect Americans from a growing array of threats,” the spokesperson added.